Early commercial production of maltol at the end of the last century involved the destructive distillation of wood. Then, in 1945, Schenck and Speilman obtained maltol by alkaline hydrolysis of streptomycin salts (J. Am. Chem. Soc. 67: 2276). The same group of investigators reported the first synthesis of maltol two years later (Spielman, M. A., and Freifelder, M., J. Am. Chem. Soc. 69:2908); in the process, pyromeconic acid derived from comenic acid, a fermentation product, was alkylated. A superior modification of this preparation was developed by Tait and Miller (U.S. Pat. No. 3,130,204).
Because of the increasing commercial importance of .gamma.-pyrones, several other syntheses were subsequently reported in the chemical and patent literature. For example, a five-step procedure employing methyl alcohol as a starting material to generate an epoxy ketone which was then refluxed with water and a Dowex.TM. 50 ion exchange resin to yield .gamma.-pyrone was briefly described by Shono and Matsumura in a short paper in 1976 (Tetrahedron Letters 17:1363). Brennan, et al., disclosed a one-pot method for preparing .gamma.-pyrones that same year (U.S. Pat. No. 4,082,717 and its divisionals, U.S. Pat. Nos. 4,323,506, 4,147,705, and 4,387,235). The process involved contacting 1(2-furfuryl)-1-ethanol in aqueous solution with two equivalents of a halogen oxidant at room temperature and then heating until the hydrolysis of the 4-halo-dihydropyran intermediate was substantially complete. Other .gamma.-pyrones such as ethyl maltol were prepared in an analogous manner from appropriate alcohols.
The investigators later published another synthetic scheme employing an O-methyl derivative of isomaltol, available in a simple two-step procedure from lactose (Weeks, P. D., et al., J. Org. Chem., 1980, 45:1109). Trimethoxyfurfturyl alcohol was prepared in good yield via a bromination-reduction sequence, and this intermediate was converted under hydrolysis conditions directly to maltol. The paper summarized other slightly different preparative schemes involving related series of reactions using similar starting materials and intermediates to those described above (id., page 1109).
Following either synthesis or extraction from natural sources, .gamma.-pyrones are typically recovered by crystallization, continuous extraction and/or co-distillation. Brennan, et al., for example, suggested extraction with chloroform followed by distillation and then crystallization from methanol (Example 1, column 6, lines 23 to 27). In Jap. Pat. Pub. No. 47-29365, Takaishi, et al., suggested using a basic ion-exchange resin to separate maltol from an aqueous solution employing ion exchange. Following this step wherein maltol is bound to the resin by ionic interaction, it is released from the column by employing common regeneration methods for anion exchangers, more specifically, by using NaOH. The step is followed by chloroform extraction. Harada and Iwasaki also suggested a chloroform extraction (Agric. Biol. Chem. 47:2921-2922 (1983)).
Though these procedures seemed promising in benchtop tests, on a commercial scale solvent extractions tend to be expensive because of the costs of the apparatus and solvents required. At lower temperatures, maltol is only somewhat soluble in water, so that large amounts of solvents are needed to extract it from the dilute aqueous solutions obtained from synthetic preparations or natural isolates. Moreover, residual organic solvent in the products are not desirable in the manufacture of food grade ingredients.
Alternative procedures such as distillations are hampered because maltol easily volatilizes with steam. It is therefore extremely difficult to obtain by concentration of aqueous solutions such as those obtained from natural source isolations or synthetic preparations. In some procedures incorporating crystallization steps, special equipment is required, and obtaining a pure product can take hours, thus increasing processing costs.
It would be desirable to have other, more economical methods of extracting and purifying maltol and other .gamma.-pyrones, particularly methods that do not utilize organic solvents.